The present invention relates to a warning or alarm system for protecting a perimeter fence or structure against cutting through or climbing over same by an intruder. More particularly, the present invention relates to such a system for a fence or structure wherein a shielded electrical cable is attached to the structure and the minute flexing of the cable when an attempted intrusion occurs is detected to provide an alarm to indicate whether a cut-through type or a climb over type of intrusion is being attempted.
When an insulated electrical cable is flexed, or when pressure is applied thereto, the resulting stress produced in the previously uncharged dielectric material of the cable by the movement results, due to the triboelectric effect, in the generation of a very small electric field signal which may be sensed with appropriate sensing circuitry. When such a cable is attached to a flexible wall structure or fence, for example a chain-link fence, minute flexing of the cable due to vibration of the fence results in the generation of an electric field signal corresponding to these vibrations. However, electric field signals will be produced both by fence vibrations, and hence electrical cable vibrations, which are desired to be detected, i.e. vibrations caused by attempted intrusions, as well as by fence vibrations which are not desired to be detected, i.e. vibrations from extraneous sources, such as wind, nearby freight trains and trucks, etc. Since the detection of these extraneous source caused signals could lead to false alarms, special signal processing is required in order to distinguish signals originating from intruder related vibrations from those originating from extraneous source related vibrations.
In order to provide adequate protection for a wall or fence, it is desired to know whether an intruder is attempting to cut through the fence or is attempting to climb over same. In general, the signals resulting from attempts to cut through a fence are of short duration, are abrupt, and are generally repeated a number of times within a predetermined short period of time. On the other hand, signals corresponding to attempts to climb over a flexible fence generally have longer duration in that they have a lower base frequency than cut-through types vibration signals and persist for a longer period of time.